1. Field of the Invention
The present invention relates to an image forming apparatus for forming an electrostatic latent image on a photoreceptor by a combination of a high electric potential, an intermediate electric potential and a low electric potential and making both an electrostatic latent image portion of the high electric potential and an electrostatic latent image portion of the low electric potential visible to thereby form an image.
2. Description of the Related Art
An image forming apparatus for forming an electrostatic latent image on a photoreceptor by a combination of a high electric potential, an intermediate electric potential and a low electric potential and making both an electrostatic latent image portion of the high electric potential and an electrostatic latent image portion of the low electric potential visible to thereby form an image is well known by JPA-48-37148, U.S. Pat. No. 4078929, etc.
In such an image forming apparatus, a potential pattern of an electrostatic latent image is often explained as a rectangular pattern on the basis of a high electric potential (VH), an intermediate electric potential (VM) and a low electric potential (VL) and lines for connection thereof as represented by the solid line in FIG. 5. However, the light intensity of a laser beam used for recording such an electrostatic latent image has a Gaussian distribution, so that an actual electrostatic latent image is recorded in the form of a curved potential pattern as represented by the broken line in FIG. 5. In the electrostatic latent image of the potential pattern represented by a broken line, when development is performed in the condition that a contrast potential between the high electric potential (VH) and a development bias (VBH) is set to be equal or substantially equal to a contrast potential between a development bias (VBL) and the low electric potential, the line width (L1) of an image obtained by normal development in an electrostatic latent image portion containing the high electric potential (VH) and the line width (L2) of an image obtained by reversal development in an electrostatic latent image portion containing the low electric potential (VL) are different from each other by a value in a range of from about 50 .mu.m to about 60 .mu.m though the difference varies in accordance with a beam diameter. There was therefore a tendency that the quality of an image by normal development was so weak with a narrow line width while the quality of an image by reversal development was so dull with a widened line width.